Flight plan deviations:

• Any track deviation (no flight plan amendment required) ⇒ Adjust the heading as soon as practicable.
• TAS +/-5% or more ⇒ Inform ATC.
• Change in time estimate +/- 3 minutes or more. ⇒ Inform ATC.
• Any altitude deviation (no flight plan amendment required). ⇒ Adjust the altitude.

SA CIVIL AVIATION REGULATIONS 
PART 91: GENERAL AVIATION AND OPERATING FLIGHT RULES
91.03.4 Air traffic service flight plan and associated procedures

(12) In the event of a controlled flight inadvertently deviating from its current ATS flight plan, the following action shall be taken 

(a) if the aircraft is off track, action shall be taken forthwith to adjust the heading of the aircraft to regain track as soon as practicable; 

(b) if the average true airspeed at cruising level between reporting points varies, or is expected to vary, from that given in an ATS flight plan by approximately five per cent of the true airspeed, the responsible ATSU shall be so informed;

(c) if the estimated time at the next applicable reporting point, flight information regional boundary, or aerodrome of intended landing, whichever comes first, is found to be in error in excess of three minutes from that notified to the responsible ATSU, a revised estimated time shall be notified to such ATSU as soon as possible; or 

(d) if the aircraft deviates from its altitude, action shall be taken forthwith to correct the altitude of the aircraft.